Betelgeuses elusive co-star left a trail of clues for astronomers
For more than a century, Betelgeuse has looked like a star with a secret.
It swells and fades on a six-year rhythm that never quite made sense. The star, about 650 light-years away in space, is old, bloated, and unstable, but not unstable enough to explain its slow pulse.
The answer might be simpler than expected. Betelgeuse, pronounced "Beetlejuice" just like the Tim Burton film, appears to have company, traveling within its own atmosphere.
Astronomers have previously speculated that the red supergiant, once thought to be on the brink of supernova, has a companion. Another star circling it could explain its strange changes in brightness.
Though researchers continue to seek a direct detection of the star that will prove its existence beyond a shadow of a doubt, scientists are celebrating new evidence from NASA's Hubble Space Telescope. A team led by Andrea Dupree, a Harvard-Smithsonian Center for Astrophysics scientist, observed ripples in Betelgeuse’s atmosphere, like the wake from a speed boat, caused by a passing star.
If this new finding, soon to be published in The Astrophysical Journal, holds up to further scrutiny, it would change how astronomers understand one of the sky’s most famous stars. Betelgeuse would no longer be thought of as a solitary giant drifting toward its final collapse. It would be part of a binary system — a Goliath with its own David: a star smaller than the sun is having an outsize impact on the giant.
The companion star, recently dubbed Siwarha, solves a major problem for scientists, said Dupree, who presented the research at the 247th American Astronomical Society meeting in Phoenix this week.
"Betelgeuse is spinning too rapidly for a supergiant. It should slow down as it becomes large, but it has a fast rotation," she told Mashable in an email. "The companion is believed to have sped up Betelgeuse by transferring angular momentum to the big star."
Angular momentum is the measure of an object's spinning power, which remains steady unless something pushes or pulls on it.
On its own, Siwarha is nothing too dramatic. Perhaps it's a red dwarf, dim and diminutive compared to Betelgeuse.
"The companion is thought to be a low-mass star," said Dupree, estimating its mass as somewhere between half and 1 ½ times that of the sun. "There are no X-rays detected, so it is not a black hole or neutron star or accreting young star. By the way stars evolve, it is not thought to be a white dwarf, either."
A NASA-led team announced it got a picture of the companion with the Gemini North Telescope in Hawaii, but the complicated imaging technique the researchers used left room for doubt, Dupree said.
What makes the star remarkable is its location. Siwarha appears to orbit so close that it never leaves Betelgeuse’s atmosphere. It sails through a turbulent sea of hot gas that extends far beyond the star’s visible surface.
As it moves, the companion leaves a trail. Gas piles up behind it, thickens, heats, and slowly expands across Betelgeuse’s face. From Earth, that wake obscures the star, reducing the light it sends out. Halfway through the orbit, when the wake has peaked, Betelgeuse is at its faintest point. Then the gas thins out, the star brightens again, and the cycle continues.
The entire star seems to react to the pattern. The surface, the atmosphere, and the surrounding gas all change on the same six-year schedule. That kind of coordination is hard to explain with solar storms alone. A companion provides a single, consistent source.
The team detected Siwarha’s wake by tracking changes in Betelgeuse's starlight over eight years.
The significance goes beyond Betelgeuse itself. If confirmed, this would be one of the clearest examples of catching a star as it spirals into its companion. Astronomers think many stars die this way, but the brief phase is easy to miss.
The companion is currently hidden behind Betelgeuse, lost in the glare. But astronomers expect it to reappear in 2027 as its orbit carries it back into view.